Increasing restrictions on the amount and types of volatiles which may be released in work environments and the desire to reduce energy consumption have prompted the development of radiation curable coating compositions which are essentially free of volatile solvents that must be evaporated during the curing of the composition. These solvent-free coating compositions are known as 100 percent reactive systems; that is, each component of the composition reacts and becomes incorporated into the cured coating when the composition is exposed to radiation.
The radiation curable coating compositions of the prior art typically contain a radiation reactive oligomer or resin; a radiation reactive diluent; a photoinitiator; and, optionally, a radiation reactive crosslinker. The radiation reactive diluent serves the function of reducing the viscosity of the oligomer or resin in order that the composition, in the uncured state, has a viscosity such that it can easily be applied as a film to a substrate using conventional techniques of the coatings art.
Virtually any monomer or oligomer which can be polymerized by a conventional thermally initiated polymerization reaction can be employed as one of the radiation reactive components of the prior art. From a commercial standpoint, however, compounds containing acrylyl or methacrylyl groups have become by far the most widely used components of radiation curable coating compositions because of the ease and rapidity with which the acrylyl or methacrylyl groups undergo radiation-induced polymerization. Monofunctional monomeric acrylate or methacrylate esters are generally employed as the radiation reactive diluent; monomeric polyfunctional acrylate or methacrylate esters are employed as the crosslinking agent; and oligomers or resins containing one or more acrylyl or methacrylyl groups are employed as the oligomer or resin component.
The acrylyl or methacrylyl oligomer or resin component can be chosen from among many different types. From the standpoint of producing a coating having good physical properties in the cured state, one of the most widely used types of oligomer or resin is a urethane oligomer or resin which is characterized in that it contains at least one urethane linkage and an average of at least one acrylyl or methacrylyl group per molecule.
While the acrylyl or methacrylyl bearing compounds which have been widely employed in radiation curable coating compositions are excellent in terms of their radiation responsiveness, and they produce cured coatings having good physical properties, their use requires that special precuationary procedures be followed to prevent workers from coming in contact with some of the materials or their vapors due to the generally high levels of toxicity associated with the materials. This is most especially true of the monomeric acrylate or methacrylate diluent component, since it is usually the lowest molecular weight component and therefore the most likely component to produce vapors. Moreover, when the cured coatings are to be employed in applications where they are to come in contact with the skin, even a small amount of residual unreacted acrylyl or methacrylyl monomer being present in the cured coating is a hazard, since unreacted monomer can migrate to the coating surface. One can mention, as typical of the compounds which have been employed as the monomeric acrylate or methacrylate diluent component, 2-hydroxyethyl acrylate, glycidyl acrylate, 2-phenoxyethyl acrylate, 2-methoxyethyl acrylate, 2-ethylhexyl acrylate, isodecyl acrylate, 2-(N-methylcarbamoyloxy) ethyl acrylate, dicyclopentyl acrylate, and the like, or the corresponding methacrylates.
The radiation curable coatings art is desirous of finding a low toxicity diluent component which can be employed partially or completely in lieu of the monomeric acrylate or methacrylate compounds of prior art compositions. To be commercially practical, such a component should be an efficient viscosity reducing agent in admixture with the other components of the composition; should not create toxic vapors or migrate out of the cured coating; and should not sifnificantly increase cure time.